Character and classification of Mollusca.

General Characters of Mollusca:

Mollusca ranges from limpets clinging to the rocks, to snail which crawl or dig or swim, to bivalve which anchor, burrow or bore, to cephalopods which torpedo through water or lurk watchfully on the bottom. They feed on every possible food and vary in size from giant squids and claims to little snails, a millimeter long. They form one of the most definitely characterized groups of animals. They have at least two characters 'radula and mantle' not found elsewhere.Some of the major characters of molluscs are as follows.

• Terrestrial or aquatic (freshwater or marine).
• Tissue-system grade of body organization.
• Body division into a head, mantles, foot and visceral mass.
• S
  • velopment direct or through free larval forms.

  Classification:

  hell, when present, usually univalve or bivalve, constituting an exoskeleton, internal in some.
• Coelom reduced and represented mainly by a pericardial cavity, gonadal cavity, and kidney.
• Triploblastic, coelomate, unsegmented and bilaterally symmetrical.
• Digestive system complete with a digestive gland or liver a rasping organ, the radula, usually present.
• The circulatory system mainly of the closed type, but some emptying into sinuses; heart with one or two auricles and one ventricle: blood with amoebocytes and hemocyanin.
• Respiration directs or by gills or lungs or both.
• The nervous system of paired ganglia, connectives and nerves. Ganglia usually form a circumcenter ring.
• Sense organ includes eyes, statocysts and receptors for touch, smell, and taste.
• Excretion by paired metanephridia (kidney).
• Dioecious or monoecious; one or two gonads with gonoducts, opening into renal ducts or to the exterior.
• Fertilisation external or internal; de

Mollusca is classified into six classes according to their symmetry and the characters of food, shell, mantle,gills, nervous system, muscles, and radula.

Class I. Monoplacophora

(Gr., monos, one+plax, plate+pherein, bearing).

1. Body bilaterally symmetrical, with a dome-shaped mantle.
2. Marine.
3. Internal segmentation.
4. Flattened limpet-shaped shell with spirally coiled protoconch.
5. Foot broad and flat, with 8 pairs of pedal retractor muscles.
6. 5 pairs of gills in pallial grooves.
7. 6 pairs of nephrides, 2 of which are gonoducts.
8. Radula in a raddualr sac; intestine much coiled.
9. Heart of two pairs of auricles and a single ventricle.
10. Nervous system with longitudinal pallial and pedal cords.

Example: Neopilina galatheae.

Class 2. Amphineura

(Gr., amphi, both+ neuron,nerve)

1. Marine.
2. The body is elongated with reduced head.
3. Redulla present.
4. Shell as 8 dorsal plats or as spicules.
5. Foot ventral, large, flat and muscular.
6. Non-ganglionated nerve rings around the mouth with 2 pairs of interconnected nerve cord.
7. Fertilisation external.

Subclass 1. Aplacophora

1. Body worm-like with a mental but no shell and foot.
2. Calcareous spicules buried in the cuticle.
3. Radula simple; mantle cavity posterior, some with a pair of bipectinate ctenidia.

Example: Neomenia, Nematomeina, Chaetoderma.

Subclass 2.Polyplacophora

1. Dorso-ventrally flattened body; small head (no eyes and no tentacles); radula, mantle,foot and external gills present.
2. Mantle cavity posterior.
3. Shell as 8 calcareous dorsal plates.

Order 1. Lepidopleurina

1. Valves of the shell without insertion plates.
2. Ctenidia a few and posterior.

Example: Lepidopleurus.

Order 2. Chitonida

1. The valve of the shell with insertion plates.
2. Gills along the whole length of mantle groove.

Example: Chaetopleura, Chiton

Class III. Scaphopoda

(Gr., scapha, boat+podas, foot)

1. Marine.
2. Tusk-shells.
3. The body within a tubular shell, open or both ends.
4. No head; mouth with tentacles; no eyes.
5. Foot conical, radula present; no gills.
6. Kidneys paired; gonads single.
7. Dioecious; larva trochophore.

Example: Dentalium, Cadulus.

Class IV. Gastropoda

(Gr.,gaster, belly+podas, foot)

1. Mostly marine; some terrestrial or freshwater.
2. Shell present or absent; univalve and usually coiled.
3. Snails and slugs.
4. Torsion of body mass at some time in development.
5. Foot flat and large.
6. Well developed head with eyes and tentacles.

Subclass 1. Prosobranchia(=Streptoneura)

1. Marine freshwater or terrestrial.
2. Body mass torted.
3. Head with a single pair of tentacles.
4. Shell closed by an operculum borne on foot.
5. Two ctenidia in mantle cavity situated anteriorly to the heart.
6. Sexes separated; gonad single; larvae trochophore or veliger.

Order 1. Archaeogastropoda(=Aspidobranchia)

1. One or two bipectinate ctenidia.
2. Two kidneys with heart with two auricles.
3. The nervous system not concentrated, with the pedal cord.
4. Shell usually coiled.
5. Genital products conveyed to outside through right kidney,
6. Fertilisation external.

Example: Fissurella (key-hole limpet), Patella (limpet), Trochus (topshell), Acmaea (limpet).

Order 2. Mesogastropoda(=Pectinibranchia)

1. Mostly marine some terrestrial.
2. One auricle, one kidney and one mono-pectinate ctenidium.
3. Syphon, operculum, and penis present; osphradium developed.
4. A nervous system without pedal cords.
5. Fertilization internal; larva usually a free- swimming veliger.

Example: Pila (apple snail),

Order 3. Neogastropoda(=Stenoglossa)

1. Shell with a short to very long siphonal canal.
2. Nervous system concentrated.
3. Osphradium large.
4. Free-swimming veliger suppressed.

Example: Buccinum, Oliva.

Subclass 2. Opisthobranchia

1. Shell small without operculum or no shell.
2. Body mass torted or detorted.
3. Gills posterior to heart.
4. One auricle, one kidney, and one gonad.
5. Exclusively marine.

Order 1. Cephalaspidea

1. Shell moderately developed.
2. Head with the tentacular shield.
3. Lateral parapodia lobes prominent.

Example: Acteon.

Order 2.Anaspidea

1. Shell usually reduced or internal.
2. Well developed parapodia lobes.
3. Head with a pair of rhinophores.

Example: Aplysia

Order 3. Pteropods

1. Shell absent or present.
2. Parapodial fins for swimming.
3. Head with a pair of rhinophores.

Example: Client, Corolla.

Order 4. Sacoglossa

1. Shell present or absent.
2. Pharynx suctorial.

Example: Elysia.

Order 5. Acochlidiaceae

1. A minute or small sized; no shell; no gills.
2. Inhabit coarse sand.

Example: Unela.

Order 6. Notaspidea

1. Shell external or reduced and internal.
2. Mantle, but no mantle cavity.

Example:Pleurobranchus.

Order 7. Nudibranchia

1. Various dorsal outgrowths.
2. No shell, gills, mantle cavity and osphradium.

Example: Aeolis.

Order 8. Pyramidellacea

1. Shell typically spirally coiled.
2. Semi-parasitic.

Example: Pyramidella.

Order 9. Philinoglossacea

1. Small naked snails without gill and head appendages.
2. Visceral mass separated from foot only by a groove.

Example: Sapha.

Order 10. Rhodopacea

1. Vermiform snails, without external appendages.
2. Anus on a right side of a body.

Example: Rhodope.

Order 11. Onchidiacea

1. Slug-like without a shell and with the pulmonary sac.
2. Posteriorly located anus.

Examples: Onchidium.

Order 12. Parasita

1. Endoparasitic in holothurians.
2. Shelled embryos.

Example: Entoconcha.

Subclass 3. Pulmonata

1. Decorated body mass .
2. With or without the shell.
3. Hermaphrodites; mostly freshwater or terrestrial, a few marine.

Order 1. Basommatophora

1. One pair of non-invaginable tentacles.
2. Eyes at tentacular bases.

Example: Siphonaria.

Order 2. Stylommatophora

1. Two pairs of imaginable tentacles.
2. Second pair of tentacles with eyes at their tips.

Example: Helix.

Class V. Pelecypoda (=Bivalvia, Lamellibranchia)

(Gr., plexus, batchet+podos, foot)

1. Body enclosed in a bivalve shell and laterally compressed.
2. No head, tentacles, eyes, jaws and radula.
3. Mostly marine, a few terrestrial.
4. Mostly filter-feeding.
5. Usually dioecious, veliger or glochidium.

Order 1. Protobranchia

1. Gills filament not folded.
2. Style sac with crystalline style.

Example: Nacula.

Order 2. Fillibranchia

1. Gills filaments reflexed but incompletely fused.
2. Style sac with crystalline style.

Example: Pesten

Order 3. Eulamellibranchia

1. Style sac short.
2. Gills filaments reflexed fused completely to form tissue sheets.

Example: Unio.

Order 4. Septibranchia

1. Gills absent.
2. Marine.
3. Stomach lined by chitin; style or sac.

Example: Poromya.

Class 4. Cephalopoda (=Siphonopoda)

(Gr., kephale, head +podas, foot)

1. Body elongated dorsoventrally.
2. Marine and free swimming.
3. Dioecious; development direct.
4. Shell external, internal or absent.
5. Head distinct and large with well-developed eyes; foot as tentacles and syphon; radula present.

Subclass 1. Nautiloidea (=Tetrabranchia)

1. Shell external, coiled or straight; without complex sutures.
2. Two pairs of gills; two pairs of nephridia.

Example: Nautilus.

Subclass 2. Smmonoidea

1. Extinct.
2. Shell external and coiled with complex septa.

Example: Pachydiscus.

Subclass 3. Coeloidea (=Dibranchia)

1. Shell internal or absent.
2. Tentacles a few with suckers.
3. One pair of gills, one pair of nephridia.

order 1. Decapoda

1. Ten arms- two elongated and called tentacles, and 8 short arms.

Example: Sepia, Loligo.

Order 2. Octopoda

1. Body globular and without fins.
2. Eight equal arms.

Example: Octopus, Argonauta.

Reference:

Aggarwal Sarita. A Text Book of Biology.,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

https://en.wikipedia.org/wiki/Mollusca